YARN CONSISTING OF REINFORCING FIBRE FILAMENTS AND A RESIN, PREFORM AND A METHOD FOR PRODUCING THE PREFORM

Abstract

EPOXY RESIN IMPREGNATED YARN AND THE USE THEREOF FOR PRODUCING A PREFORM Abstract A yarn consisting of reinforcing fibre filaments and a resin that is infiltrated into the yarn and can be repeatedly melted and solidified by cooling to room temperature, whereby the filaments of the yarn are at least partially bound to one another by the resin, whereby the yarn contains 2.5 to 25 wt.% of infiltrated resin relative to its total weight, and whereby the infiltrated resin consists of a mixture of at least two epoxy resins E1 and E2, E1 having an epoxy value in the range of 2000 to 2300 mmol/kg of resin and E2 having an epoxy value in the range of 500 to 650 mmol/kg of resin, and the weight ratio E1 :E2 of the epoxy resins E1 and E2 in the mixture being so chosen that the infiltrated resin mixture has an epoxy value between 550 and 2100 mmol/kg of resin, a preform comprising the yarn, a method for producing the preform and its use in producing a composite are described.

Full Text

Description
The present invention relates to a yam, the use of the yarn to produce a preform, a preform comprising the yarn, a method for production of the preform, and its use in the production of a composite.
A known method of producing fibre-reinforced plastic components consists in first preparing a preform similar in form to the component, transferring the preform to a tool having the form of the component, and finally producing the desired component by addition of a matrix resin.
In contrast to preforms, prepregs (preimpregnated fibres) already contain both components (fibres and matrix resin) in the same ratio as in the final mixture, and are therefore resistant to bending, even in the form of the semi-finished product. To prevent premature unwanted reactions, this material must also be stored cool and even then has only a limited shelf life. On account of the material's flexural rigidity and its production in the form of large-width rolls, the use of prepregs is restricted to components of large surface area that are nearly planar. The presence of the matrix resin does not permit textile processing or wrinkle-free application of prepregs around narrow radii or on strongly contoured geometries, for example.
If flexible yarns are used for the production of preforms, it is possible to produce surfaces that are far more heavily curved. Moreover, the maximum shelf life of

the preform is significantly improved as compared with prepregs, because the matrix resin is not added until the production of the composite material.
JP 2003 003376 A describes a carbon fibre bundle for production of a prepreg. The bundle comprises 20000 to 100000 filaments and is held together by a sizing agent containing polyoxyalkylene groups and epoxy groups. The carbon fibre bundle contains 0.5 to 3 wt.% of sizing agent relative to its total weight.
DE 27 46 640 A describes a carbon-fibre reinforced material on which a resin mixture containing an epoxy resin, a condensation product of an acid component and a hydroxyl component, and an oxyalkylene derivative of a phenol has been applied. The carbon fibres contain 0.01 to 10 wt.% of resin mixture relative to their total weight.
DE 39 42 858 A describes a sizing agent for carbon fibres that contains an epoxy resin and a nonionic emulsifier containing tertiary amino groups, at least one functional group capable of reacting with the epoxy resin, and at least one group with emulsifying action. 0.3 to 10 wt.% of sizing agent is said to be present on the fibre.
DE 20 33 626 A describes a method for production of a prepreg from mechanically high-grade fibre materials such as, in particular, fibres of glass, boron or carbon. The fibres are impregnated with a solution containing a curable polyadduct that contains free glycidyl groups and consists of triglycidyl isocyanurate, a curing agent and an organic solvent.
DE 201 20 447 U1 describes a preform consisting of a textile flat structure, such as a fabric orfibrewoven fabric layer, with a thermoplastic, non-crosslinked binder, which can be, for example, an epoxide, adhering to the surface of the textile flat structure. According to DE 201 20 447 U1, the binder is applied by, for example, spraying a binder solution on to the surface of the textile flat structure, whereby the binder

solution contains powder particles that are uniformly dispersed in a solvent, and whereby the powder particles can consist exclusively of epoxy resin.
However, when a preform is produced from a textile flat structure, the binder coating is unevenly distributed over the thickness of the textile flat structure, and in some places, particularly where the yarns are in contact with one another, is almost absent. A preform of this type therefore often has low slip resistance, so that further handling of the preform is difficult or even impossible.
Additional disadvantages arise if the preform is to have perforations, which, if the preform is produced from textile flat structures, must in many cases be produced by cutting out material. This requires additional processing time, and the offcuts result in considerable loss of material and generate a corresponding amount of waste. This method for producing a preform is therefore cost- and labour-intensive, which is reflected in the costs of the composites produced from it.
The object of the present invention is therefore to at least reduce the disadvantages described above.
The object of the invention is achieved by a yarn consisting of reinforcing fibre filaments and a resin, infiltrated into the yarn, that can be repeatedly melted and solidified by cooling to room temperature, whereby the filaments of the yarn are at least partially bound to one another by the resin, whereby the yam contains 2.5 to 25 wt.% of infiltrated resin relative to its total weiqht, and wherebv the infiltrated resin consists of a mixture of at least two epoxy resins E1 and E2, E1 having an epoxy value in the range of 2000 to 2300 mmol/kg of resin and E2 having an epoxy value in the range of 500 to 650 mmol/kg of resin, the weight ratio E1 :E2 of the epoxy resins E1 and E2 in the mixture being so chosen that the infiltrated resin mixture has an epoxy value between 550 and 2100 mmol/kg of resin.

Because the filaments of the yarn are at least partially bound by the resin, the yarn of the invention has extremely good compactness.
In the yarns of the invention, the resin should be chosen so that the yarn coated with it is not tacky at room temperature. In general, therefore, a yam of this type can not only be wound up, but can also be stored in the wound-up state with maintenance of its textile properties and unwound again even after long periods of storage. For example, the yarn of the invention can be unwound without any problem after 12 months of storage, and then shows values of strength, elastic modulus and elongation at break, as measured by the methods of DIN 65 382, that have changed only marginally, if at all.
Finally, the yarn of the invention can be used to produce a preform without the need for complicated addition of further binder material, whereby, however, the binding between the yarns is better than in a state-of-the-art preform. Moreover, in contrast to state-of-the-art production of the initially described preform with perforations, no wastage of yarn occurs. The need to dispose of yarn offcuts therefore does not arise.
In a preferred embodiment, the yarn of the invention contains 3 to 10 wt.% of infiltrated resin relative to its total weight.
In another preferred embodiment of the invention, the reinforcing fibre filaments of the yarn of the invention are carbon fibre filaments that have been obtained from precursors of pitch, polyacrylonitrile or viscose, or aramid filaments, glass filaments, ceramic filaments, boron filaments, synthetic fibre filaments or natural fibre filaments, or a combination of one or more of these filaments.
The object is further achieved by a yarn consisting of reinforcing fibre filaments and a resin that is infiltrated into the yarn and can be repeatedly melted and solidified by cooling to room temperature, whereby the filaments of the yarn are at least partially

bound to one another by the resin, whereby the yarn contains 2.5 to 25 wt.% of infiltrated resin relative to its total weight, whereby the reinforcing fibre filaments are electrochemically pretreated carbon filaments, whereby the infiltrated resin contains at least two bisphenol A epichlorohydrin resins H1 and H2 in the weight ratio H1:H2 of 1.1 to 1.4, H1 having an epoxy value of 1850 to 2400 mmol/kg and a molecular weight of 800 to 1000 g/mol and being solid at room temperature, and H2 having an epoxy value of 5000 to 5600 mmol/kg and a molecular weight of
In a preferred embodiment, the yarn of the invention contains 3 to 10 wt.% of infiltrated resin relative to its total weight.
In another preferred embodiment, the reinforcing fibre filaments of the yarn of the invention are carbon fibre filaments that have been obtained from precursors of pitch, polyacrylonitrile or viscose, or aramid filaments, glass filaments, ceramic filaments, boron filaments, synthetic fibre filaments or natural fibre filaments, or a combination of one or more of these filaments.
The reinforcing fibre filaments are preferably carbon fibre filaments.
In regard to yarns made of carbon fibre filaments, a yam pretreated by electrochemical oxidation is particularly appropriate. This yarn can consist of several thousand, preferably of about 3 000 to 24 000, filaments, and especially preferably of 3 000 to 12 000 filaments.
For infiltration of the resin into the yarn, any technique is appropriate in principle that supports rapid and complete wetting of the reinforcing fibre filaments of the yarn by

the resin. Methods of this type are described in, for example, EP 1 281 498 A. For example, the yarn can be sprayed with a resin dispersion. Alternatively, a film of the resin dispersion can be applied to a smooth roller or in the grooves of a roller, and the yarn drawn over the smooth roller or through the grooves of the roller. The yarn is preferably conducted through a bath containing the resin dispersion.
For the resin dispersion mentioned above, any liquid mixture that forms a stable dispersion with the resins used in the invention is in principle suitable as the liquid phase. The liquid mixtures that are particularly suitable from the viewpoint of emission protection are those that are aqueous and have a low VOC (volatile organic content). For example, a mixture of water and an alcohol such as 2-propoxyethanol has proved advantageous for the bisphenol A epichlorohydrin epoxy resin that is preferred in the method of the invention.
In a particularly preferred embodiment, the yarn of the invention contains a total of 0.5 to 1.7 wt.% of H1 and H2, and 2.0 to 4.3 wt.%, respectively 2.3 to 5.5 wt.% of H3, relative to its total weight.
The production of the yarn of the invention can be integrated into the production process of the yarn to be infiltrated with resin after the yarn has been dried and before it is wound up, whereby the dried yarn can be infiltrated with the at least one resin either individually or in the form of a warp sheet, whereby, if a resin dispersion is used for the infiltration, a yarn tension of 0.25 to 1.3 cN/tex allows good wetting of the reinforcing fibre filaments.
The quantity of the infiltrated resin, i.e. 2.5 to 25 wt.% relative to the total weight of the yarn, required to be applied for the invention can of course be arrived at by adjusting the speed at which the yarn is, for example, conveyed through a bath containing the dispersion of one or more of the resins described above, the immersion length, and

the resin concentration in the bath. The speed at which the yarn is conveyed through the bath lies preferably in the range of 120 to 550 m/h, and especially preferably in the range of 150 to 250 m/h. The immersion length lies preferably in the range of 0.2 to 1 m. The resin concentration in the dispersion relative to the dispersion weight lies preferably in the range of 2 to 35 wt.%, and especially preferably in the range of 2 to 7 wt.%.
A drying temperature in the range of 140 to 330°C has proved particularly suitable for drying of the yarn coated with the resin dispersion described above.
The yarns of the invention can advantageously be used to produce a preform.
The object of the present invention is further achieved by a preform comprising the yarns of the invention as described above, whereby the yarns are bound to one another by the infiltrated resin at points of mutual contact.
Although the yarns of the invention also allow production of fabrics that yield, after melting and resolidification of the infiltrated resin, a preform that is slip resistant to a particularly high degree, it is advantageous to produce the preform of the invention from the yarns of the invention because in doing so the yarns can be positioned in the direction in which mechanical stresses are expected to be highest during the use of a composite produced from the preform of the invention.
For example, in a preferred embodiment of the preform of the invention, the yarns are positioned unidirectionally, so that the preform can be further processed into a composite, during the use of which the mechanical load is expected to be highest in exactly this direction of the yarns.
In a further preferred embodiment of the preform of the invention, the yarns are positioned bi-, tri- or multidirectional^, so that the preform can be further processed

into a composite, during the use of which the mechanical load is expected to be highest in exactly these two or more directions of the yarns.
In addition to the above mentioned flat embodiments of the preform of the invention, the uni-, bi-, tri- or multidirectional^ positioned yarns can be wound around a body having, for example, a cylindrical form, so that a three-dimensional preform results.
Moreover, an embodiment of the preform of the invention is preferred in which the yarns are present as short yarns that can be oriented in all spatial directions. This preform is therefore particularly suitable for production of a composite in the use of which mechanical stresses can arise in all spatial directions.
The object of the present invention is further achieved by a method for producing a preform comprising the following steps:
a) provision of one of the yarns of the invention,
b) positioning of the yarn in a configuration corresponding to the configuration of the desired preform,
c) heating of the configuration resulting from step b) to a temperature above the melting temperature of the resin infiltrated in the yarn, and
d) cooling of the configuration resulting from step c) to at least below the melting temperature of the resin.
In a preferred embodiment of the method of the invention, the configuration resulting from step b) is compacted simultaneously with the heating in step c).
The preform of the invention, or produced by the method of the invention, shows high slip resistance because the yams of the preform of the invention are bound to one another by the infiltrated resin. The preform of the invention is therefore easily

handled, which is advantageous particularly in its subsequent processing to a composite.
If the preform of the invention, or produced by the method of the invention, is to have perforations, these can be realised by appropriate positioning of the yarns, that is, without any loss of material arising from offcuts. The cost- and labour-intensive tailoring described above for the state of the art is thereby unnecessary, and thus no waste is produced. The production of a composite with perforations is therefore easier and more economical.
Furthermore, in the production of the preform of the invention, or the preform produced by the method of the invention, the use of one of the yarns of the invention instead of a textile flat structure allows the yarn to be positioned in the direction in which mechanical stresses are expected to be highest during the use of the subsequently produced composite.
For example, in a preferred embodiment of the method of the invention for producing a preform, yarns of the invention are unidirectionally positioned in step b), so that following step d) a preform of the invention results in which the yarns are unidirectionally positioned.
In a further preferred embodiment of the method of the invention for production of the preform of the invention, the yarns of the invention can be positioned in step b) in bi-, tri- or multidirectional layers in a configuration corresponding to the configuration of the desired preform. The yarns of the invention can be used exclusively for this purpose. Alternatively, only some of the yarns within a layer may be yarns of the invention, the remainder consisting of yarns whose filaments have no resin coating. The yarns configured as described are heated in step c) of the method of the invention to a temperature above the melting temperature of the resin with which the

yarns are infiltrated, with simultaneous compaction of the yarns if required. This makes the yarns tacky. After cooling in step d) to at least below the melting temperature of the resin, a preform of the invention results in which the yarns are positioned bi-, tri- or multidirectionally.
In a further embodiment of the method for production of the preform of the invention, the yarns of the invention are cut into short pieces of length, for example, 1 to 1000 mm, and preferably 1 to 40 mm, and the short pieces of yarn are placed in a mould in step a). In step b) of the method of the invention, the short pieces of yarn are then heated to a temperature above the melting temperature of the resin with which the yarns are infiltrated, making the short yam pieces tacky, with simultaneous compaction of the yarns if required. After cooling to at least below the melting temperature of the resin in step d), a preform of the invention results wherein the yarns of the invention are present as short yarns with isotropic orientation.
The preform of the invention, or produced by the method of the invention, or resulting from the use of the invention, can be used advantageously, for the reasons described above, to produce a composite comprising a matrix selected from one of the groups of polymers, metals, ceramics, hydraulically setting materials, and carbon, whereby thermoplastics such as polyethylene imine, polyetherketone, polyetheretherketone, polyphenylene sulfide, polyethersulfone, polyetherethersulfone and polysulfone or duromers such as epoxides are suitable as the polymer matrix; steel (alloys) or titanium, for example, as the metal matrix; silicon carbide or boron nitride, for example, as the ceramic matrix; mortar or concrete as the hydraulically setting materials; and, for example, graphite as the carbon matrix.
In the composites resulting from the uses of the invention, the yarns of the invention are positioned in the direction in which the highest mechanical stresses are expected during use of the composites. The use in accordance with the invention of the yarns

of the invention and of the preforms produced from these therefore lead to composites in which the orientation of the yarns is tailored to the mechanical stresses expected.
Analytical Methods
The epoxy value of the expoxy resins used in the method of the invention is determined in accordance with DIN 53188 of October 1975.
The tensile strength and tensile modulus of the composite reinforced with a preform of the invention are measured as in EN 2561 -B.
The invention will now be described in more detail with-the help of the following examples.
Example 1: Production of a Coated Yarn
A yarn of carbon fibre filaments with a yam count of 400 text is conducted in the dry state at the rate of 240 m/h and with a yarn tension of 340 cN through a first bath of temperature about 20°C. The bath contains an aqueous dispersion containing two bisphenol A epichlorohydrin epoxy resins H1 and H2. The weight ratio of H1 and H2 is 1.2. H1 has an epoxy value of about 2200 mmol/kg and a molecular weight of 900 g/mol, and is solid at room temperature. H2 has an epoxy value of 5400 mmol/kg and a molecular weight of
The yarn infiltrated with H1 and H2 is conveyed directly afterwards through a second bath containing a dispersion of a bispheno! A epichlorohydrin epoxy resin with a melting temperature range of 120 to 130°C, a number average molecular weight of 2870 g/mol and an epoxy value of 515 mmol/kg. The dispersion medium consists of a mixture of 76 wt.% water and 24 wt.% 2-propoxyethanol. The concentration of the epoxy resin in the bath is 4 wt.%. The residence time of the yarn in the second bath is a few seconds. On leaving the bath, the yarn, now infiltrated also with H3, is dried by passing it firstly through a vertically positioned drier at 300°C and then through a horizontally positioned drier at 330 C°. The resulting yarn contains 4.3 wt.% of infiltrated resin H1+H2+H3, whereby the infiltrated resin has an epoxy value of 1300 mmol/kg.
Example 2: Production of a Preform
The coated yarn of Example 1 is wound onto a metal plate, each of whose surfaces (of dimensions 280 mm x 300 mm) is covered with a release film, by means of a laboratory winder (with yarn speed 23.1 mrn/s and yarn traction force 400 cN), the winding being carried out in each case up to the edge of the metal plate. A wound layer (with fibre mass per unit area of 267 g/m2) oriented at 90° to the winding axis is first produced on both sides of the metal plate. The metal plate is then rotated through 90° in such a way that the existing wound layer lies parallel to the winding axis. In the next step, a further wound layer oriented at 90° to the winding axis is superposed on the existing wound layer, using identical winding conditions. In this way a layered structure with yarn orientations of 0° and 90° respectively results on both sides of the metal plate. The winding procedure described above is repeated until four wound layers, with yarn orientation alternating between 0° and 90°, lie superposed on each side of the metal plate.

Each of the wound layers on the two sides of the metai plate is then covered with a release film. The metal plate, complete with the two four-layer windings and the release films, is then tempered in a press for 1 hour at a surface pressure of 2 bar and a temperature of 125°C.
The resulting pressing is allowed to cool to below the melting temperature of the resin. The two wound packages are then cut apart at the end surfaces of the metal plate and the four release films removed. This results in two dimensionaiiy stabie preforms, each with a four-layer alternating 0° and 90° structure, i.e. with a bidirectional positioning of the yarns.
Example 3: Production of a Composite
The preform produced in Example 2 is cut to a size of 200 x 200 mm and processed in the usual way into a composite with a 4-layer alternating 0° and 90° yarn structure, using RTM6 resin from the company Hexcel that has previously been heated to 80°C and is applied in such a quantity that a composite with a fibre volume content of 60% can result. The composite is identical in form to the preform used.
Example 4: Production of a Preform with Biaxial Positioning of the Yarns
Together with a standard carbon fibre filament yam available under the name Tenax HTA 5131 400tex f6000 tO from Tenax Fibers GmbH, Germany, the yarn produced in Example 1 is fed to a biaxial layering process, whereby every fourth parallel yarn is one from Example 1.
The biaxial arrangement has the following configuration:
In the first layer, the standard carbon fibre filament yarns and the yarns of Example 1 are at an angle of 45° to the direction of production.

In the second layer, lying immediately above the first layer, the standard carbon fibre filament yarns and the yarns from Example 1 are at an angle of minus 45° to the direction of production.
The resulting configuration is heated to 125°C by bringing it into contact for 2 minutes with a hot calender heated to 145°C, and is then cooled to room temperature, so that a bonded textile flat structure results (preform in state A). A composite produced from this as in Example 3 with a fibre volume content of 60% has a tensile strength of 1100 MPa and a tensile modulus of 70.5 Gpa, both measured in the main direction of the fibres.
Example 5: Production of a Preform with Three-Dimensional Positioning of the Yarns
The state A preform produced in Example 4 is moulded, by means of a spherically shaped tool in the form of a hemisphere of diameter 150 mm and consisting of male and female moulds, in a press for one hour with a surface pressure of 2 bar and a tool temperature of 125°C, into a geometry corresponding to that of the tool. After being cooled to room temperature, the resulting preform is hemispherical in shape.

Claims
1. A yarn consisting of reinforcing fibre filaments and a resin, infiltrated into the yarn, that can be repeatedly melted and solidifed by cooling to room temperature, whereby the filaments of the yarn are at least partially bound to one another by the resin, whereby the yarn contains 2.5 to 25 wt.% of infiltrated resin relative to its total weight, and whereby the infiltrated resin consists of a mixture of at least two epoxy resins E1 and E2, E1 having an epoxy value in the range of 2000 to 2300 mmol/kg of resin, and E2 having an epoxy value in the range of 500 to 650 mmol/kg of resin, the weight ratio E1:E2 of the epoxy resins E1 and E2 in the mixture being so chosen that the infiltrated resin mixture has an epoxy value between 550 and 2100 mmol/kg of resin.
2. Yarn according to Claim 1, whereby the yarn contains 3 to 10 wt.% of infiltrated resin relative to its total weight.
3. Yarn according to Claim 1 or 2, whose reinforcing fibre filaments are carbon fibre filaments that have been obtained from precursors of pitch, polyacrylonitrile or viscose, or ararnid filaments, glass filaments, ceramic filaments, boron filaments, synthetic fibre filaments or natural fibre filaments, or a combination of one or more of these filaments.
4. Yarn consisting of reinforcing fibre filaments and a resin, infiltrated into the yarn, that can be repeatedly melted and solidified by cooling to room temperature, whereby the filaments of the yarn are at least partially bound to one another by the

resin, whereby the yarn contains 2.5 to 25 wt.% of the infiltrated resin relative to its total weight, whereby the reinforcing fibre filaments are electrochemically pretreated carbon filaments, whereby the infiltrated resin contains at least two bisphenol A epichlorohydrin resins H1 and H2 in the weight ratio H1.H2 of 1.1 to 1.4, H1 having an epoxy value of 1850 to 2400 mmol/kg and a molecular weight of 800 to 1000 g/mol and being solid at room temperature, and H2 having an epoxy value of 5000 to 5600 mmol/kg and a molecular weight of
5. Yarn according to Claim 4, whereby the yarn contains 3 to 10 wt.% of infiltrated resin relative to its total weight.
6. Yarn according to Claim 4 or 5, whose reinforcing fibre filaments are carbon fibre filaments that have been obtained from precursors of pitch, polyacrylonitrile or viscose, or aramid filaments, glass filaments, ceramic filaments, boron filaments, synthetic fibre filaments or natural fibre filaments, or a combination of one or more of these filaments.
7. Yarn according to one or more of Claims 4 to 6, characterised in that the yarn contains a total of 0.5 to 1.7 wt.% of H1 and H2, and 2.3 to 5.5 wt.% of H3, relative to its total weight.

8. Use of the yarn according to one or more of Claims 1 to 3 or of a yarn according to one or more of Claims 4 to 7 to produce a preform.
9. Preform comprising yarns according to one or more of Claims 1 to 3 or yarns according to one or more of Claims 4 to 7, whereby the yarns are bound to one another by the infiltrated resin at points of mutual contact.
10. Preform according to Claim 9, characterised in that the yarns are positioned unidirectionally.
11 .Preform according to Claim 9, characterised in that the yarns are positioned bi-, tri- or multidirectionally.
12.Preform according to Claim 9, characterised in that the yams are present as short yarns.
13. Method for producing a preform according to one or more of Claims 9 to 12, comprising the following steps:
a) provision of a yarn according to one or more of Claims 1 to 3 or of a yarn according to one or more of Claims 4 to 7,
b) positioning of the yarn in a configuration corresponding to the configuration of the desired preform,
c) heating of the configuration resulting from step b) to a temperature above the melting temperature of the resin infiltrated in the yarn, and
d) cooling of the configuration resulting from step c) to at least below the

melting temperature of the resin.
14. Method according to Claim 13, characterised in that the configuration resulting
from step b) is compacted simultaneously with the heating in step c).
15. Use of the preform according to one or more of Claims 9 to 12 to produce a
composite comprising a matrix selected from one of the groups of polymers,
metals, ceramics, hydraulically setting materials, and carbon.
16. Use of the preform produced by the method of Claim 13 or 14, or resulting from
the use of Claim 8, to produce a composite comprising a matrix selected from one
of the groups of polymers, metals, ceramics, hydraulically setting materials and
carbon.